By Maria Kielmas | Updated March 24, 2022
Gold is famously resistant to corrosion, but halogens—especially chlorine—can dissolve it. In industrial practice, sodium hypochlorite (bleach) combined with hydrochloric acid generates chlorine gas, which reacts with gold embedded in ore to form soluble gold chloride. This technique, once the first commercial gold extraction method, can be replicated in a laboratory setting with appropriate safety precautions.
Grind the gold ore to a fine sand‑like consistency using a mortar and pestle. Transfer the pulverized material into a plastic bowl or bucket.
In a clean flask, mix 35 % hydrochloric acid with sodium hypochlorite bleach in a 2:1 acid‑to‑bleach ratio. Use a volume at least six times the amount of ore. The reaction is exothermic and releases chlorine fumes, so wear a face mask and perform the operation outdoors or in a well‑ventilated fume hood.
Pour the acid‑bleach mixture over the ore in the plastic bowl, stirring gently. Allow the solution to react for four hours, stirring every 20 minutes. Gold will dissolve into the liquid as gold chloride. Filter the mixture through a standard or coffee filter to remove solids, and collect the filtrate in a clean flask.
Dissolve powdered sodium metabisulfate in water to produce a sodium bisulfate solution. Add this to the gold chloride filtrate and let the mixture sit for four hours. During this period, gold chloride reacts with bisulfate to form insoluble gold particles.
Observe a brown precipitate forming at the bottom of the flask. Decant the liquid and dry the remaining wet gold powder by heating gently on a stove until all moisture evaporates.
Transfer the dry gold powder to a crucible or melting dish. Using an oxy‑butane torch, heat the material from the edges toward the center until it reaches approximately 1,947 °F (1,060 °C). Once fully melted, remove the heat source and allow the gold to cool undisturbed. The resulting solid can then be cast into desired shapes.
Always wear gloves when handling acids and chlorine gases. Perform the mixing of acid and bleach outdoors or in a fume cupboard, as the reaction is exothermic and releases toxic chlorine gas.
Chlorine is a hazardous gas; conduct all procedures in a well‑ventilated area or under a fume hood to protect yourself from inhalation exposure.
